The present invention relates to a method for preparing carbon fibers from a pitch by melt-spinning the pitch, infusibilizing the pitch fibers with a mixture of air and a gaseous oxidizing agent under a circulating gas of a velocity in a specified range and carbonizing the thus treated pitch fibers into the carbon fibers.
In the preparation of the carbon fibers, the pitch fibers are subjected to a treatment of infusibilization of the pitch fibers before the carbonization thereof. The pitch fibers are made infusible when the pitch fibers are subjected to a take-up system in which the pitch fibers are fed and taken up around a roll, or a net-belt conveyer system in which the pitch fibers are loaded on and transferred by a net-belt conveyer. However, in the take-up system, a high productivity cannot be obtained since the pitch fibers cannot be taken up at a high velocity due to the low physical strength and ductility thereof. Moreover, it takes much time to mend the pitch fibers when it is broken during the operation.
In the well-known net-belt conveyer system, the pitch fibers are formed into waves by the net-belt and the fibers are locally deformed by the meshes of the net, because the pitch fibers are not sufficiently infusibilized in the steps in the infusibilizing furnace.
The inventor had found that these problems are solved by providing a bar on the upper part of a tray having a u-type cross section and introducing the tray having the pitch fibers suspended on a bar into the furnace of infusibilization and of carbonization thereby effectively carrying out the infusibilization and carbonization (refer to U.S. Pat. No. 4,265,869).
However, in the tray system, the height of suspension of the fiber is restricted by the strength of the fiber, and the packing density of the fiber cannot be made so much large because of the necessity of preventing the accumulation of heat generated by the infusibilizing reaction and the necessity of uniforming the gas flow for the sufficient replacement of the generated gas in the carbonization. The ordinary value of packing density is 1 to 20 kg/m.sup.3. Such an amount of the packing density of the fiber is too small from the economical view. That is, in order to make the capacity of the total facilities, it was necessary to have larger furnaces of infusibilization and of carbonization. This was not desirable from the consideration of raising the production efficiency.